NASA says its Kepler Space Telescope has found the first Earth-size planet orbiting in the "habitable zone" of a distant star, with the potential to possess liquid water critical for life.

Part of a five-planet system orbiting a cool dwarf star, Kepler-186f provides confirmation Earth-size planets do, in fact, exist within the habitable zone of stars other than our own sun, NASA researchers say.

All previous planets detected within such zones have been larger than Earth -- at least 40 percent bigger -- which makes determining their makeup and surface conditions difficult, they say.

Kepler-186f is much closer to Earth in size, although its composition and mass are currently unknown.

"The discovery of Kepler-186f is a significant step toward finding worlds like our planet Earth," says Paul Hertz, NASA's Astrophysics Division director at the agency's headquarters in Washington.

"Future NASA missions, like the Transiting Exoplanet Survey Satellite and the James Webb Space Telescope, will discover the nearest rocky exoplanets and determine their composition and atmospheric conditions, continuing humankind's quest to find truly Earth-like worlds."

The Kepler system of planets is about 500 light-years away from us, with planets, including the Earth-size one, orbiting a star classed as an M dwarf, also termed a red dwarf, about half the mass and size of our sun. It is in the constellation Cygnus.

Stars of this type account for almost three-quarters of the stars in our Milky Way galaxy, scientists say.

"M dwarfs are the most numerous stars," says Elisa Quintan at NASA's Ames Research Center in California. "The first signs of other life in the galaxy may well come from planets orbiting an M dwarf."

Kepler-186f's "year" as it orbits is 30-days long and it gets about one-third the amount of energy from its parent star that Earth receives from the sun, the researchers say.

"Being in the habitable zone does not mean we know this planet is habitable. The temperature on the planet is strongly dependent on what kind of atmosphere the planet has," says Thomas Barclay, research scientist at the Bay Area Environmental Research Institute at Ames. "Kepler-186f can be thought of as an Earth-cousin rather than an Earth-twin. It has many properties that resemble Earth."

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